1. Field of the Invention
The invention relates to an interior finish for a basin designed to come into contact with an aqueous medium.
2. Description of the Prior Art
The principal interior finishes in the swimming pool industry and related industries are pool plaster or marcite. Plaster and marcite are normally composed of white cement, aggregates such as crushed marble and limestone, and white silica sand. The ratio of white cement to aggregates is generally 1 part cement to 1-1/2 or 2 parts aggregates. The plaster or marcite, which may further include a set accelerator (calcium chloride) and a pumping aid (bentonite), is mixed with water and applied to the interior of a swimming pool, spa or other water basin by hand or with a pump.
Although plaster and marcite have been in use for over 40 years, certain shortcomings have caused their popularity to decline. Thus, plaster and marcite tend to stain, etch, scale and show other forms of mineral precipitation. Etching of plaster and marcite may result in the appearance of generally rounded spots having a size of about 1/8 to 3/4 inch. These spots occur in the surface layer usually referred to as the "creme" or laitance layer. The spots are porous, and hence permeable, and are due to the selective dissolution of calcium compounds from the surface layer.
The incidence of spot etching has been increasing, and the increase is often attributed to the increased use of acidic swimming pool and spa sanitizers such as trichloro-s-triazinenrine and bromochlorodimethylhydantoin. These sanitizers have grown in popularity because they are long-lasting and easy to use.
The growing popularity of acidic sanitizers has been accompanied by a trend towards softer water. This is water from which minerals and divalent metallic compounds have been removed to improve palatability. However, by decreasing the concentrations of these components, and particularly the concentrations of calcium bicarbonate, calcium carbonate and other calcium salts, the carbon dioxide-bicarbonate equilibrium is shifted such that the dissolution of calcium compounds increases.
Air pollution also has a deleterious effect on plaster and marcite. Thus, certain pollutants in the air react to form acid rain which, in turn, causes the water in pools, spas and other water basins to become more aggressive.
The above phenomena are mostly limited to the surfaces of plaster and marcite finishes. Beginning in the mid to late 1980s, the swimming pool industry began experimenting with exposed aggregate surfaces. This type of surface is produced by mixing cement and water with small rounded pebbles or other small aggregates and applying the mixture to the interior of a pool, spa or other water basin. After troweling the mixture in traditional fashion, the surface creme or laitance layer is removed by misting or washing and is subsequently pumped out of the pool as a slurry. The slurry is fed into a containment vessel or holding tank and subsequently transported from the job site.
An exposed aggregate surface has an appearance similar to exposed concrete except that thousands of pebbles or other aggregates (both natural and synthetic) project outward. Such a surface, while rougher than conventional plaster or marcite, has major advantages over plaster and marcite. To begin with, the absence of a surface creme and the presence of thousands of projecting pebbles, beads or other aggregates make the damage from aggressive water less visible. Furthermore, the irregular appearance of an aggregate surface also hides many types of surface stains as well as the normal mottling or shade variations which occur in traditional plaster and marcite.
To give the consumer a choice of colors, differently colored pebbles or other aggregates, as well as colored pigments, are used in exposed aggregate surfaces. The pigments, which are mostly mineral pigments in the form of inorganic metal oxides, have a greater effect than the aggregates and provide the most attractive finishes.
For proper coloration, a pigment must be uniformly dispersed in the mixture to be used for an exposed aggregate surface. The mixtures for exposed aggregate surfaces are generally produced at the job site and, due to the manner in which pigments are added, color inconsistencies often arise. Moreover, pigmented surfaces of darker coloration tend to exhibit "fading" over time. This is usually the result of carbonation of the exposed cement binder around the pebbles, beads or other aggregates.
Another problem with pigmented surfaces occurs when a small field repair must be performed. Even when given the initial formulation, it is extremely difficult for field technicians to properly match repaired areas and original areas.
A further shortcoming of exposed aggregate surfaces is the effect they produce at the water line. Traditional pool plaster and marcite must be continuously submerged during curing because plaster or marcite cured in air ultimately undergoes a volume change which can result in cracking and possible eventual delamination. To allow complete submersion of the plaster or marcite while curing, ceramic tile is placed around the perimeter of the pool, spa or other water basin at the water line. Once the plaster or marcite has been applied, the pool, spa or other water basin is filled with water to the level of the tile so that the plaster or marcite is fully submerged for curing.
With exposed aggregate surfaces, the tile can be eliminated since the volume change during hydration in the air is reduced thereby decreasing the likelihood of cracking and delamination from the concrete or shotcrete substrate.
Severe mineral buildups develop at the water line with plaster and marcite as well as with exposed aggregate surfaces. In a pool, spa or other water basin lined with plaster or marcite, these buildups form on the ceramic tile and can be removed fairly easily by different methods. However, with an exposed aggregate surface, the mineral buildups develop on the aggregate surface and are extremely difficult to remove. This is partly attributable to the nature of the interfacial transition zone, that is, the interface between the bulk cement binder and the aggregates. This specialized zone is composed of duplex layers of calcium hydroxide and calcium silicate hydrate which, upon reacting with minerals in the water, produce particularly tenacious buildups of calcium salts and other salts.
U.S. Pat. Nos. 5,650,004 and 5,588,990 disclose cement compositions containing one or more pozzolans. Among other things, these compositions are intended to increase resistance to attack by aggressive water. In these compositions, the pozzolan or pozzolans generally have a particle size of 7 micrometers or more.
It is known that the interfacial transition zone between the bulk cement binder and the aggregates is approximately 50 micrometers thick. See, for instance, Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete: Proceedings--Fourth International Conference, Istanbul, Turkey, May 1992--"Effect of Mineral Admixtures on the Cement Paste Interface", Article SP 132-136, pp. 656-669. This publication further discloses that the interfacial transition zone is a weak link in cementitious materials and that the thickness of the zone can be decreased, while the density is increased, by the addition of finely divided pozzolans. The publication additionally discloses that finely divided pozzolans reduce the amount of soluble calcium hydroxide in the interfacial transition zone. Among the pozzolans studied in the publication are metakaolin having a mean grain size of 1.5 micrometers and silica fume having a mean grain size of 0.1 micrometer.
The use of silica fume to increase the resistance of cement to chemical deterioration is also taught by T. A. Durning et al. See "Using Microsilica to Increase Concrete Resistance to Aggressive Chemicals", Concrete Admixtures, American Concrete Institute Compilation 22nd, P.15.
In spite of the advances made to date, compositions for use in aqueous media are not entirely satisfactory. For instance, interior finishes made from such compositions tend to hydrate rapidly in hot weather which makes it difficult to trowel and wash the finishes. Moreover, significant numbers of shrinkage cracks still tend to develop in the finishes.